1. Field of the Invention
The present invention relates to a method and an apparatus for preparing crystalline thin-films for solid-state lasers, and more particularly to a method and an apparatus for preparing crystalline thin-films for solid-state lasers by which such crystalline thin-films for solid-state lasers which can be used for the development of micro-cavity and the like, and allowed solid-state laser to directly grow on a semiconductor laser can be prepared.
2. Description of the Related Art
Recently, in order to respond a variety of needs for shorter laser wavelength, expansion in wavelength band to be oscillated, higher output power in laser, higher efficiency in laser and higher quality in beam and so on, extensive studies have been made with respect to research for novel solid-state laser materials, and higher quality in the solid-state laser crystals than have been heretofore known.
Heretofore, solid-state laser crystals have been prepared by the flux method, crystal pulling method or Verneuil's method and the like method. These methods will be described in brief hereinbelow.
Flux Method
In this method, crystal is deposited and grown from a molten material at a high temperature, and a molten inorganic salt or an oxide functions as its solvent.
Crystal Pulling Method
In this method, a seed crystal is dipped in molten liquid or saturated solution to be compatible with each other, and then the seed crystal is gradually pulled up to grow a single crystal on the extreme end portion of the seed crystal.
Verneuil's method
In this method, a raw material which has been finely pulverized is blown into high temperature flame such as oxyhydrogen flame and the like to heat and melt the raw material, and the molten material is received by a seed crystal so as to grow a single crystal thereon.
As described above, in any of conventional methods, first a material intended to be a solid-state laser crystal is molten at high temperatures, and then either a crystal is deposited and grown, or a single crystal is allowed to grow on the seed crystal.
While the above described flux method, crystal pulling method or Verneuil's method and the like are very effective methods for preparing a large bulk crystal, it is difficult to achieve uniform dispersion of active ionic species intented to be the emission subject of a laser beam, as a result of spatial control in laser host crystal, and prevention of inclusion of ionic species intended to be the absorption subject of laser beam into laser host crystal is difficult.
More specifically, since solid-state laser crystal is allowed to grow in a rod-like shape in the prior art, temperatures are different in the central portion of the rod-like solid-state laser crystal as compared to from the peripheral portion thereof, and as a result diffusion conditions of ion in both the portions are different. Accordingly, it is difficult to effect uniform doping of the active ionic species intended to be the emission subject along the axial and diametrical directions of solid-state laser crystal. For this reason, uniform dispersion of active ionic species in the laser host crystal could not have been attained heretofore.
Furthermore, since it is difficult to control valence number of the ion to be doped as active ion species, there have been problems where in the ion which had been doped did not become the emission subject of laser beam, but rather became the ion species which is absorption subject of laser beam. For example, in such a case where Al.sub.2 O.sub.3 is used as laser host crystal, and doping is effected by using Ti as active ionic species, when Ti is trivalent, it becomes the emission subject, but when Ti is di- or tetravalent, it becomes the absorption subject. As described above, it was extremely difficult to control such valence number in the prior art.
Moreover, as described above, while in the prior art is preferable to prepare a large bulk crystal, it is difficult to prepare thin-film crystal. Accordingly, it was difficult to control film thickness on an atomic scale.
In the above described prior art, a solid-state laser crystal is prepared by the manner quite different from the conventional semiconductor production process. Thus, it was impossible to grow a solid-state laser crystal on a semiconductor substrate in a high-vacuum vessel containing the semiconductor substrate which is used for semiconductor production process. For this reason, solid-state laser crystal heretofore could not be produced in combination with semiconductor and semiconductor laser.